1. Field of the Invention
The present invention relates to an imaging apparatus, imaging method, imaging program, and storing medium, more particularly though not exclusively, including an automatic focusing function having multiple focal point detecting regions.
2. Description of the Related Art
Heretofore, the following arrangements have been discussed as a range-finding frame display method of an imaging apparatus. First, the subject distances of subjects to be displayed on multiple regions within a photographing screen are each measured, using an external range-finding sensor. Subsequently, determination is made regarding whether or not subjects other than an in-focus subject are included in the range of depth of field. Subsequently, the subject included in the depth of field is displayed on a screen such that the photographer can view the subject (e.g., Japanese Patent Laid-Open No. 05-232371).
Also, a conventional rear focus lens system included in a video camera, has a relationship between subject distance, focal length, and focus lens position where a focus lens can bring a subject image into focus (hereinafter, referred to as “focusing lens position”).
FIG. 9 is a graph illustrating the relationship between subject distance, focal length, and focusing lens position. The multiple curves shown in FIG. 9 illustrate the relationship between the focal length and the focus lens position at each certain subject distance.
The rear focus lens system maintains a focused state when a zoom lens performs zooming operation by moving the focus lens in accordance with the curve corresponding to the focal length of the zoom lens.
Further, a conventional imaging apparatus has been discussed which uses multiple focal point detecting regions. First, multiple focal point detecting regions are set within a photographing screen, and a focal point detection is made in each focal point detecting region, thereby detecting a focusing lens position for each region. Subsequently, one focusing lens position is eventually determined. (e.g., Japanese Patent Publication No. 07-072764).
However, with the conventional imaging apparatus, the difference of focusing lens positions caused due to the difference between subject distances becomes smaller as the focal length becomes shorter, and becomes greater as the focal length becomes longer (see FIG. 9). The depth of focus in the case of a short focal length is around one half of the depth of focus in the case of a long focal length (see FIG. 10). Accordingly, the depth of focus does not proportionally become less deep as the focal length becomes shorter near the shorter side of the focal length (see FIG. 10).
Consequently, because the difference in focusing lens position between the far and near subject distance decreases when going from the mid focal length value to the short focal length value, the ratio of the depth of focus to the difference of focusing lens positions caused due to the difference between subject distances becomes greater as the focal length becomes shorter. Thus, the range of a subject distance included in the depth of focus becomes wider, i.e. more of the subject distance positions (e.g., positions between near and far) lie within the depth of focus. Contrarily, the ratio of the depth of focus to the difference of focusing lens positions caused due to the difference between subject distances becomes smaller as the focal length becomes longer. Thus, the range of subject distance included in the depth of focus becomes narrower.
Now, for example, let us consider a case where, with a photographing screen including range-finding frames 51a, 52a, and 53a serving as focal point detecting regions such as shown in FIG. 11, photographing is performed using a picture composition in which another subject (pillar in FIG. 11) is present at a position (e.g., around 2 m behind the main subject) in the range-finding frame 52a. In this case, when the focal length is short, the difference between focusing lens positions of the focus lens caused due to the difference between subject distances (e.g., the distances between the pillar and the main subject) is small (see FIG. 9). Accordingly, there is the possibility that all of the focusing lens positions of the range-finding frames 51a, 52a, and 53a are included in the depth of field or the depth of focus (see FIG. 12), in which case the photographer cannot determine which subject the focus lens has focused upon.